You also may want to look at overwintering onions, which are planted from seed now in 6b. Varieties include Topkeeper from Territorial and Bridger and Desert Sunrise from Johnny's, among others. They would be phenomenal in an unheated high tunnel. The thinnings make excellent scallions, and the mature onions bulb up in May and cure like a dream. Look at back threads on overwintering onions, a most interesting and rewarding crop!

Onion SETS, if fall planted, will bolt, especially the larger ones. B/C they are practically ONIONs and have passed one cycle of their life and the next cycle is to flower and produce seed. . I have planted them in the past, both purchased from nurseries and grocery stores (sold as Pearl Onions)
I think better option is planting seedlings in the spring.

Thanks so much, Planatus. Plan to order a few of your suggestions. Last fall, I also broke up several clumps of chives and had phenomenal reproduction and enjoyment of these ...and still do. I had never had such huge and tender chives before from these plants that I have had for decades.

Seysonn, the onion sets' growth in our HT last fall did not behave as those in your experience. It seemed as though the HT going into fall/winter was absolutely perfect. Only 4-5 bulbs out of hundreds had a flower stalk. Even those were tasty in fritattas. I let them bloom to see what would result. One had black seeds which I will test for germination in a week or so. Some folks say that growing zones make a huge difference in alliums in High Tunnels, while others say it is more of a light availability that determines the production of the fall-winter crop. Some suggest recording light availability by automatic methods, but I am not so advanced and am still learning how to assess soil moisture levels by inexpensive/simple tests. Our central KY site had abundant fall-winter-spring sunlight this first trial.

Have sent emails to a few HT gurus in hopes of learning more about allium possibilities. One person in MN-zone 3-said that by using agribon and a plastic sheet over wire hoops, he managed green onions thru the winter-alas, variety not specified. He also did not say if onions were grown thru the 1 ml black plastic.

Have not yet located a USA High Tunnel(unheated) index that covers varietal issues for all zones for all vegetables. Not even all states have a listing yet, from my research. Our NRCS HT has only a single 6ml cover, so it's results may be varied compared to the double layered HT covers used by many commercial growers.

This fall, plan to do a short row with garlics, elephant garlic and one with yellow potato onions, as well as the winter onions planatus mentioned. Will do a similar test planting outdoors in a raised bed. We eat alliums at least twice a day-so can never have too many!